There has been an incident on London Bridge, London's Metropolitan Police said Saturday night.
"We are dealing with an incident on #LondonBridge, when we have more information we will update this twitter feed."
A witness, Mark Roberts, told CNN that he was on the bridge when a van came swerving down the roadway at a high rate of speed.
The van hit several people, knocking one person about 20 feet into the air, he said. It swerved into oncoming lanes before hitting a bus stop and coming to a stop, Roberts said.
"Within my line of sight, there were five or six people on the ground that were not moving," he said. "It looked to me that the van was aiming at the people."
Roberts said he heard what sounded like gunshots a brief time later. He estimated 100 people were on the bridge at the time.
Both lanes of the bridge were blocked, police said.
Authorities have not said what caused the incident.
The US embassy in London tweeted: "Please avoid the area and monitor local news/@metpoliceuk for updates."
England has been on edge since May 22, when a suicide bomber killed 22 people at an Ariana Grande concert at the Manchester Arena.
The bridge incident also comes more than two months after a man drove an SUV into a crowd on the sidewalk along Westminster Bridge in London, killing at least four people.
After ramming the car into a barrier outside the Houses of Parliament, the driver got out and stabbed a police officer to death. The attacker was gunned down by a police officer.
The assailant, Khalid Masood, 52, of West Midlands, reportedly had a criminal record and may have had connections to violent extremism, British Prime Minister Theresa May said.
Developing story - more to come